The polarization multiplexing system can simultaneously transmit two independent signals (h and v) in the same bandwidth by using two orthogonal polarization states (x and y) of the light, thus increasing twofold the transmission efficiency of the channel. However, in the wavelength division multiplexing system, as compared to a single polarization system, the nonlinear effect between channels in the polarization multiplexing system has even greater influence. As found by the inventors during the process of studying the present invention, the nonlinear effect between channels (i.e. cross-phase modulation) may cause nonlinear phase noise and nonlinear polarization crosstalk, of which the nonlinear polarization crosstalk is the principal factor that leads to damage of nonlinearity between channels in the wavelength division multiplexing system employing polarization multiplexing modulation, while the study on nonlinear polarization crosstalk caused by cross-phase modulation is currently few and far between.